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1.7 Basic Modes of sendmail | 27
DATA tells the remote host to ???get ready.??? The remote machine says to send the message,
and the local machine does so. (The message is not printed as it is sent.) A dot
on a line by itself is used to mark the end of a mail message. This is a convention of
the SMTP protocol. Because mail messages can contain lines that begin with dots as
a valid part of the message, sendmail doubles any dots at the beginning of lines
before they are sent.* For example, consider what happens when the following text is
sent through the mail:
My results matched yours at first:
126.71
126.72
...
126.79
But then the numbers suddenly jumped high, looking like
noise saturated the line.
To prevent any of these lines from being wrongly interpreted as the end of the mail
message, sendmail inserts an extra dot at the beginning of any line that begins with a
dot, so the actual text transferred is:
My results matched yours at first:
126.71
126.72
.... ?†? note extra dot
126.79
But then the numbers suddenly jumped high, looking like
noise saturated the line.
The SMTP-server program running at the receiving end (for example, another sendmail)
strips those extra dots when it receives the message.
The remote sendmail shows the queue identification number that it assigned to the
mail it accepted:
250 2.
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